Various techniques have been used in the past to achieve noninvasive imaging of blood flow using ultrasound. Recent developments in Doppler echocardiography are an example.
A typical ultrasound blood flow imaging system includes an ultrasonic transmit-receive transducer for transmitting ultrasonic pulses into a region of the body under diagnosis and for receiving echo signals of the transmitted ultrasound waves reflected due to blood flow in the area being scanned. A typical diagnosis with ultrasound includes scanning the patient with the ultrasound probe to measure blood flow in an artery, a vein, or in the heart. A signal processing system processes the received echo signals for measuring the Doppler shift frequency of the echo signals for use in calculating the velocity of the blood flow, and the result of the velocity distribution measurement is displayed as a Doppler blood flow image.
In order to estimate the Doppler shifts of the echoes received from the blood cells, an ultrasound imaging system commonly transmits pulses at one location in the region under diagnosis and then detects the variations in the phase of the echoes from pulse to pulse.
Echo signal components Doppler shifted by the blood flow are extracted from the Doppler signal components carrying the information of the internal moving part of the body. Typically, an MTI (moving target indication) filter (also referred to as a stationary canceller) is used to eliminate "clutter" signals reflected from stationary or slowly moving targets such as the wall of the heart or blood vessels, and only the signal components Doppler-shifted by the blood flow being measured are extracted. The MTI filter output is then typically processed in a velocity estimator to extract the Doppler frequency information which is converted to velocity data displayed in color to provide a two-dimensional image of the blood flow being measured.
The present invention is concerned with improving the flow estimation sensitivity of a Doppler color flow imaging system. The techniques provided by this invention are based on maximum entropy spectral estimation. Unlike classical power spectrum estimation techniques, this method does not suffer from the inherent "windowing" problems present in all finite-length sampled data sequences. The present method allows higher speed imaging resulting from needing fewer data samples to estimate velocity accurately. The method also provides improved velocity detection sensitivity in signals highly contaminated by noise. The invention offers reduced sensitivity to quadrature phase errors, resulting in relaxed constraints on the analog demodulator used in the system for input to the velocity estimator. The number of multiply and accumulate operations increases linearly with sequence length, as opposed to some Discrete Fourier Transform techniques which increase at an N.sup.2 rate, and this has additional advantages.